Crufts has long been considered a competition of the utmost decorum. The closest it normally comes to opprobrium is someone accidentally picking up an opponent's grooming brush. But this year, the dog show has been rocked by an unlikely betting scandal, with fears that a mystery "super dog" may be about to pad off with the top prize.

The competition starts today, but yesterday the bookmaker William Hill announced that it was closing the book on bets as to which category the competition's overall winner – the Best in Show – will come from.

The unprecedented decision came after a flood of bets predicting that the winning dog would come from the previously unfancied "utility dog" category – a miscellaneous group of breeds including bulldogs, miniature poodles and dalmatians. The market for bets opened yesterday morning with the chances of a "utility dog" winning being rated at 13/2 by odds compilers at William Hill.

They had predicted that an entrant from that category would stand little chance against dogs from the pastoral, terrier, hound, toy, gun and working dog classes. However, after just a few hours of betting, the odds on the utility dog class had been shortened to 6/4. Then, in the early afternoon, betting was closed after the price was cut to odds-on favourite of 1/2.

Rupert Adams, a spokesman for William Hill, said the company feared that there was a "super dog" in the running which could swipe the overall prize. "There must be an exceptional dog out there that we have not heard about," he said.

"Normally in betting, when we shorten the price people start to back off a little and back other contenders, but that hasn't happened here and no matter how short a price we offer on a utility dog winning, people keep betting.

"It's essentially a seven-horse race with all the money going on one horse. We decided enough was enough and closed the book."

Mr Adams said that the firm became suspicious after more than 20 bets of £100 or more were placed on the utility dog category. He added that William Hill faced a payout of more than £10,000 should that be the case.

Paddy Power, the other bookmaker offering odds on Crufts, still have their book open, but they too say they have seen a surge of bets being placed on a dog from the utility category winning.

The organisers of the event say they aren't aware of any sure-to-win super dogs. Caroline Kisko, the secretary of the Kennel Club, said: "There are 23,000 dogs in the competition ... It's impossible to say that there is one dog certain to win the top prize."

The dogs in the utility and toy categories will be judged today, with the winners returning for the Best in Show finale on Sunday.

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